1. Field of the Invention
This invention relates to a fuel injection control for an engine and more particularly to an improvement of a fuel injection control for a direct cylinder injected, multiple cylinder engine.
2. Description of Related Art
Some of internal combustion engines with multiple cylinders are provided with a fuel injection system including a feedback control system therein. The fuel injection system has a single fuel injector at an air intake manifold or a plurality of fuel injectors at respective air intake passages. The feed back control system, although executes other controls such as a firing control of spark plugs, generally controls amounts of the injected fuel to restrain dispersion of the air fuel ratio (A/F) between the respective cylinders. That is, the control is done to obtain the optimal air fuel ratio in the aggregate and eventually to improve total emissions and fuel efficiency. For this purpose, for example, an oxygen density sensor (O.sub.2 sensor) is provided at a specific cylinder. The feed back control system controls amounts of the injected fuel for respective cylinders in response to the output from the O.sub.2 sensor. More specifically, duration of opening time of control valves is controlled. The control valves are provided at nozzles of the respective fuel injectors.
In the mean time, a direct cylinder injection system is also embodied in such multiple cylinder engines. This system controls to inject lean fuel directly into combustion chambers of the respective cylinders so as to improve fuel efficiency.
FIG. 1 shows various operational ranges of the direct cylinder injected engine, which is applied to an outboard motor, in a matrix of the engine speed versus the engine load.
The engine is operated with rich fuel (A/F=11 to 12) in the range A of the low speed and low load like the idling or trolling state. Meanwhile, the engine is operated with lean fuel (A/F=15 to 16) in the range B of the middle speed and middle load. Further, it is operated with excessive rich fuel (A/F is approximately 11) in the range C of the high speed and high load. Under such operational conditions, conventionally, amounts of the injected fuel are controlled to obtain the optimal air fuel ratio.
However, as seen in FIG. 2, particularly in the range of lean A/F, i.e., the range B, changes in the air fuel ratio by the adjustment of fuel amounts exert relatively large influence to the output power of the engine because of the sensitiveness in the lean A/F set range. In addition, since the direct injected engine must employ the injection pressure that is extremely larger than that of the intake injected engine, the amount of the injected fuel per unit time is also large. This means that changes in the diameter of the control valves provided at the respective fuel injector nozzles caused by, for example, aging can give rise the difficulty in fine adjustment of the air fuel ratio. Because of these reasons, each cylinder can generate different outputs, emissions and fuel efficiency.
The difficulty in fine adjustment exists remarkably in the lean A/F set range that focused on the improvement in fuel efficiency. That is, subtle changes in the air fuel ratio cause the relatively large changes in the output power and then make the dispersion in the respective outputs of the cylinders. This results in the difficulty in optimizing emissions and fuel efficiencies at each cylinder.
The operational ranges will be described more in detail in connection with a control strategy of a preferred embodiment later.
Multiple cylinder engines for marine propulsion devices such as outboard motors generally have aggregated exhaust passages whereby exhaust gasses are discharged to the atmosphere through the body of water. Such an engine tends to be influenced by the states in the other cylinders and also the back pressure of the surrounding body of water. Particularly, the multiple cylinders of the outboard motor are disposed vertically with each other and hence each cylinder has a different height from the water surface. Accordingly, respective lengths of the exhaust passages are different to each other so that the influences of the back pressure are also different. In addition, each cylinder has a different temperature therein. Thus, the dispersions in the air fuel ratio and also in the output of the respective cylinders can easily occur.
It is, therefore, a principal object of this invention to provide an improved direct injected, multiple cylinder engine wherein the dispersions in air fuel ratio and also in outputs of the respective cylinders can be as small as possible.
In addition to that, direct injected engines that operate on a two stroke crankcase compression principle are likely to have another problem. The output characteristics of the air fuel ratio in this two stroke engine tend to shift to the rich side in comparison with an engine which operates on a four stroke principle or an engine which has an intake injection system. Because, blow-out amounts of mixture in this two stroke engine exert large influence to the air intake efficiency. The term "blow-out amounts of mixture" means amounts of unburnt charge that is formed with the injected fuel and the intake air charge induced through the scavenge port and will be discharged outside through the exhaust port.
The situation occurring in a two stroke engine will be described more in detail hereunder with reference to FIG. 3.
FIG. 3 illustrates a graphical view showing the relationship between the air fuel ratio and the output power of the engine.
In this graph, the characteristic curve identified with the reference characters a, b and c represent the outputs of the O.sub.2 sensor in a four stroke engine, a two stroke engine it, with the intake injected system and a two stroke engine with the direct injected system, respectively.
As seen in this figure, the curve a abruptly changes nearly at the theoretical air fuel ratio T.A/F. Because, all of the air charges including oxygen are almost completely burnt and exhaust gasses hardly contain any unburnt charges or air charges therein in the 29, four stroke engine. Thus, fuel amounts of the four stroke engine are relatively easily controlled in a feed-back control manner based upon the output of the O.sub.2 sensor so that the air fuel ratio is always kept in the optimal state. The unburnt charges (gasses) contain the sprayed liquid fuel as well as air charges.
However, the curve b, as seen in FIG. 3, shifts to the rich side from the theoretical air fuel ratio T.A/F. Because, in the two stroke engine with the intake injected system, unburnt charges as well as the air charges reach the position of the O.sub.2 sensor. The curve c further shifts to the rich side because only air charges reach the O.sub.2 sensor in the two stroke engine with the direct injected system.
In some outboard motors having a multiple cylinder engine, the air fuel ratio feed back control system has the O.sub.2 sensor at a specific cylinder as described above. The cylinder is a reference cylinder and only this cylinder is controlled in a feed back control manner based upon the output of the O.sub.2 sensor. Meanwhile, the other cylinders are controlled with the control amount that is adjusted to the control amount of the reference cylinder by calculation so as to be adapted to each cylinder in every running range.
In the adjustment calculation, if the change portion of the characteristic curve is shifted to the rich side like the curves b and c, conventionally, the reference cylinder is incorrectly controlled so that the air fuel ratio thereof approaches the objective air fuel ratio that is shifted to the rich side. In other words, the injected fuel will be under the excessive rich condition. Accordingly, the fuel efficiency and emissions are not improved. The other cylinders are also controlled to be at the rich side and hence cause the same problem.
This problem also occurs with a linear sensor as well as the O.sub.2 sensor.
FIG. 4 illustrates a graphical view showing the relationship between the air fuel ratio and the output of a linear sensor as shown in FIG. 3, the curve a is omitted though. The same reference characters in this figure as in FIG. 3 indicates the situations at the same engine and the injection system.
It is, therefore, another object of this invention to provide an improved direct injected, multiple cylinder engine, particularly a two stroke engine, wherein deviation from the optimal air fuel ratio, i.e., the theoretical air fuel ratio in control can be as small as possible.